For this trick you need to select Recon class and sprint to the back of the enemy base as quickly as you can, sticking to the sides of the map. Then wait behind cover watching for:

A. groups of defenders all facing one way
B. snipers facing the enemy
C. sqauds on thier way to the front

As soon as you identify one of these targets fall in behind them like your one of them (helps keep rest of team from noticing you). When you get to them pull out your knife and kill them so nobody sees another one die. I have taken out 6-8 people camping out a rooftop like this.